Add softened cream cheese, sweetened condensed milk, lemon juice and ⅔ of the lemon zest to a large mixing bowl, and beat on high until smooth and creamy (about 4 minutes).
Transfer to a graham cracker crust.
Sprinkle reserved lemon zest in top, and refrigerate until filling is set, about 3 to 4 hours.
Serve chilled, and garnish with extra lemon slices, if desired.
